This invention relates to apparatus for the wireless control of a plurality of controllable objects. More specifically, it relates to apparatus wherein an individual control unit is furnished for each of the objects but the control signals for all units are modulated in a predetermined sequence on to a high frequency carrier. Each control signal includes an address portion signifying the addressed object and a command portion which specifies the operation the object is to carry out.
Cranes, locomotives and other moveable or stationary objects are increasingly radio controlled by stationary or moveable individual control units. In order to avoid having a different carrier frequency for each object, apparatus has become known in which a plurality of independently operable objects can be controlled by control signals modulated on to a single high frequency carrier. German Offenlegungschrift 24-49-6660 discloses apparatus for the wireless control of a plurality of independently operable ojbects by means of control signals modulated in a predetermined time sequence on to a high frequency carrier. The control signals comprise an address portion signifying the addressed object and a command portion which signifies the operation to be carried out by the object. It also fixes the position within the raster of each control signal, that is it determines the sequence in which the individual control units transmit the control signals. This known arrangement has a disadvantage that gaps arise within the time raster if one or more of the individual control units become inactive. These gaps cannot be filled. The frequency with which the individual control units can transmit control signals is thus always determined by the maximum number of individual control units which may be incorporated into the time raster.
Other known apparatus is disclosed in German DT-AS-22-11-313 which also discloses apparatus for wireless control of a plurality of independently operable objects by means of individual control units. Each of the individual control units has a counter which is preset to a count signifying its position in the sequence. Each individual control unit further has a field strength detector which responds to control signals by the other units and enables all counters to count timing pulses when the control signal from another unit terminates. The first counter to reach its preset count is the one which transmits. A blocking circuit is provided to prevent transmission from any one unit more than once in the cycle. In this arrangement, gaps which arise can be filled by the subsequent units in the sequence. However, the overall cycle time is fixed because of the blocking circuit which prevents the individual unit from transmitting twice in a cycle. Further, new units which are activated for the first time or which assume radio contact with other individual control units for the first time are very difficult to incorporate into the sequence. Difficulties in synchronization arise which may cause the signal from the newly introduced individual control unit to be blocked indefinitely.
A further disadvantage of this known system is that residual time gaps arise in the sequence since each individual control unit must execute its full count following termination of the previous control signal, before being able to transmit.